1. Field of the Invention
The invention relates to control of an engine in a hybrid electric vehicle powertrain with a divided power flow path from the engine and from an electric motor.
2. Background Art
In a hybrid electric vehicle (HEV) powertrain having an internal combustion engine and an electric motor, the power of each of these power sources may be distributed through separate torque flow paths to vehicle traction wheels, one path being a mechanical path and the other being an electromechanical path. The percentage of total power transferred to the vehicle traction wheels from each power source will vary depending upon the operating conditions for the vehicle, including driver demand for power and a state-of-charge of a battery that is electrically coupled to a vehicle traction motor and to an electric generator.
In a known hybrid electric vehicle powertrain of this type, the generator may be mechanically connected to the sun gear of a simple planetary gear unit, and a carrier may be drivably connected to an engine. The ring gear of the simple planetary gear unit is connected through gearing to the traction wheels. Engine speed can be controlled by controlling generator speed as the level of electro-mechanical power delivery is adjusted due to varying vehicle operating conditions.
In co-pending application Ser. No. 11/566,876, filed Dec. 5, 2006 entitled “System and Method for Controlling an Operating Temperature of a Catalyst of a Vehicle Exhaust System,” now U.S. Pat. No. 7,832,198, which is assigned to the assignee of the present invention, a known relationship between engine torque and engine speed may be used in determining a driver demand for power at the traction wheels. A desired threshold of engine power is related functionally to a predetermined temperature of a catalyst in the engine exhaust system. Power from the engine can be modified by supplying a power bias from the battery to reduce the portion of the total power supplied by the engine needed to meet the driver demand for power at the wheels. The temperature of the catalyst thus can be maintained below a predetermined threshold catalyst temperature. This eliminates the need for reducing catalyst temperature by enabling an air/fuel enrichment feature in a powertrain of this type, which would deteriorate the quality of engine exhaust gas emissions.
An operating strategy for controlling engine speed and torque values to meet a desired overall powertrain efficiency is disclosed in co-pending U.S. patent application Ser. No. 11/161,424, filed Aug. 2, 2005 entitled “Optimal Engine Operating Power Management Strategy for a Hybrid Electric Vehicle Powertrain,” now U.S. Pat. No. 7,398,147, which also is assigned to the assignee of the present invention. That operating strategy recognizes that engine efficiency has the most influence on total system efficiency, but highly efficient engine operation is not achieved at the expense of a lowering of the total system efficiency. The appropriate engine speed and torque values for achieving maximum system efficiency is achieved by considering the efficiency of each of the components of the overall powertrain system. The power loss for each of the system components is obtained, and the engine speed is commanded so that it corresponds to the minimum value of the sum of the total powertrain system losses.
The strategies of the co-pending patent applications include method steps that are common to the method steps of the strategy of the present invention. In the case of the strategy of the '424 patent application, engine noise, vibration and harshness characteristics in a hybrid electric vehicle powertrain are not considered while effecting best powertrain efficiency. The vehicle system control architecture is not designed to adjust the target engine speed to achieve optimum noise, vibration and harshness (NVH). There is no arbitration of powertrain operating variables to achieve acceptable powertrain efficiency while meeting a goal of optimum noise, vibration and harshness.